In the semiconductor industry, critical steps in the production of integrated circuits are the selective formation and removal of films on an underlying substrate. Chemical-mechanical polishing (CMP) is widely used to reduce the thickness and planarize the topography of films on the substrate (generally a silicon wafer).
In a typical CMP process, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or rotating the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. FIG. 1 shows a conventional CMP arrangement wherein the wafer 1 is held against a polishing pad 11 using a wafer carrier 2. Wafer carrier 2, which often comprises a metal plate, is covered by a backing film 3 in contact with the backside of the wafer (that is, the side not being polished). The wafer, wafer carrier, and backing film are held in radial alignment by a retaining ring 4.
Chemical-mechanical polishing using this standard arrangement does not result in a uniform polishing rate across the wafer, and thus does not produce a planar polished surface. Both radial and non-radial variations in uniformity have been observed. A number of techniques have been employed in attempts to equalize the polishing rate at different areas of the wafer, as detailed below.
CMP tools often use vacuum or backside air pressure at the surface of the wafer carrier to hold a wafer during loading on the tool and to eject a wafer after the process is finished. This may be done by providing a porous wafer carrier plate (as described in U.S. Pat. No. 5,645,474) and pre-punching holes in the backing film. Another typical arrangement (shown in FIG. 1B) uses a wafer carrier 2 with a plenum formed therein and holes 12 aligned with holes 13 in the backing film, to conduct air to the backside of the wafer.
Another known practice is to modulate the amount of backside air pressure during the polishing process to control and improve polishing uniformity (see, for example, Murakami et al., VMIC Conference, 1996). Air pressure applied to the backside of the wafer causes the wafer to flex outward, which in turn causes the wafer center to come into closer contact with the polishing pad. Generally, additional force on the wafer at the center reduces the polishing rate near the wafer perimeter relative to that at the center, thereby improving the overall polish uniformity.
Unfortunately, the use of backside air pressure has drawbacks. If the air is permitted to leak around the edge of the wafer, a substantial portion of the applied force is lost. In addition, greater and greater amounts of backside air pressure are required as various tool elements (such as the polishing pad and backing film) degrade with repeated use.
Furthermore, since the use of backside air pressure reduces the relative polishing rate near the wafer edge, it aggravates a well-known radial non-uniformity called "edge bead." FIG. 2 shows the radial variation in polishing rate on a 200 mm wafer in a typical CMP process. The polishing rate is generally higher near the periphery of the wafer than near the center, but drops sharply at a radius of 90-98 mm. This results in a sharp increase in film thickness (a bead) at the edge of the wafer after polishing. It is generally accepted that the edge bead is caused by deflection of the polishing pad as it meets the wafer edge; this is referred to as "pad dive." As the pad moves under the wafer and wafer carrier, the wafer edge forces the pad to tilt locally. The pad pressure on the wafer is very high at the outer 2 mm of the wafer, but very low at a radial distance of 3 to 7 mm from the edge. This low pressure results in a low polish rate. This problem is aggravated by the use of a stacked pad arrangement (preferred for many processes for better overall planarization), wherein a hard polishing pad is in contact with the wafer and a soft pad is placed underneath.
Various tool modifications have been suggested to reduce the effect of pad dive. These include milling the carrier face to a predetermined concave profile (so that the perimeter of the carrier is in closer contact with the wafer) and placing shims behind the backing film in the 90-98 mm radius area. However, even if the effects of backside air pressure and pad dive are brought into balance, that balance cannot be maintained for repeated process cycles as various components of the polishing apparatus are subjected to wear.
An additional problem that appears at the wafer edge is called "slurry penetration." If the wafer is not sealed to the wafer carrier at its edge, slurry may penetrate between the wafer edge and the retaining ring and deposit on the backside of the wafer near the edge. A cleaning process is then required after the CMP process to remove the deposited slurry. This problem is aggravated by backside air leaking radially outward, which dries the slurry and causes it to adhere to the wafer (as noted by Ikenouchi et al., CMP-MIC Conference, 1999).
There remains a need for a wafer carrier and wafer backing film arrangement which provides improved polishing uniformity and is simple and inexpensive to implement on a wide variety of tools.